Two models are being used to examine the control of synthesis and proteolysis of connective tissue components in lung: (1) changes during normal neonatal lung growth and (2) examination of "compensatory" lung growth following unilateral pneumonectomy. In both models, a rapid accumulation of lung collagen is preceded by a shift in the emphasis of the proteins synthesized by lung cells so that relatively more collagen is synthesized. Preliminary studies have suggested that mechanical factors may be partially responsible for the initiation of these complex biochemical events.